Derivatives of pyridine-2,3-dicarboxylic acid, in particular 5-alkylpyridine-2,3-dicarboxylic acids, are valuable intermediates for the preparation of 2-(2-imidazolin-2-yl)nicotinic acid compounds, which have distinct herbicidal activity. In particular, the (R,S)-5-ethyl-2-(4-isopropyl-4-methyl-5-oxo-2-imidazoline-2-yl)nicotinic acid is of economic interest. The preparation of this compound is described, for example, in U.S. Pat. No. 4,460,776.
In accordance with their importance, a number of syntheses are known for the preparation of pyridine-2,3-dicarboxylic acid derivatives. According to the teaching of EP 274,379 A2, compounds of pyridine-2,3-dicarboxylic acid are obtained either by reaction of 3-halogeno-2-ketosuccinic acids with .alpha.,.beta.-unsaturated aldehydes or ketones and ammonia, or from aminobutenedicarboxylic esters and .alpha., .beta.-unsaturated aldehydes or ketones. These process either start from halogen-containing compounds, which require the use of halogen-resistant apparatuses and lead to halogen compounds polluting the environment as waste products, or from commercially unavailable or insufficiently available substances. Their use is therefore limited.
The synthesis of pyridine-2,3-dicarboxylic acid derivatives described in EP 299,362 A1 is also based on the use of aminobutenedicarboxylic esters and .alpha., .beta.-unsaturated carbonyl compounds. Both starting materials have to be used in pure form. The high expenditure for distillation resulting from this requirement casts doubt on the economy of the process.
The process of EP 308,084 A1 for the preparation of pyridine derivatives starts from halogenopropenoic or halogenobutenoic acid derivatives which are reacted with a 2-alkylacrolein oxime. An example of this synthesis is the preparation of 5-ethylpyridine-2,3-dicarboxylic acid from N-phenylchloromaleinimide and 2-ethylacrolein oxime. Apart from the fact that N-phenylchloromaleinimide can only be obtained by a complicated sequence of several reaction steps from maleic anhydride, the process requires long reaction times of up to 60 hours and gives the desired compound only in moderate yields.
As can be seen, the known processes do not fulfill all industrial and economical conditions which have to be satisfied by industrially performed processes. In some cases, they are based on the use of starting materials whose preparation is complicated or which lead to by-products polluting the environment during the synthesis, or further processing and, in other cases, they require long reaction times or give the desired compounds only in insufficient yields.
The aim of the present invention is to remove the disadvantages of the prior art and to provide a process for the preparation of esters of 5-alkylpyridine-2,3-dicarboxylic acid which satisfies the requirements of industrial practice.